1. Field of the Invention
The present invention relates generally to charged-particle beam systems, such as those used for specimen inspection, review, parametric characterization and for other purposes.
2. Description of the Background Art
Charged-particle beam systems include, for example, electron beam imaging (EBI) systems, electron stimulated x-ray (ESX) systems, and other systems. These systems are often applied to inspect, review or measure insulating samples, such as, for example, a semiconductor wafer with an insulating layer. The insulating layer on a semiconductor wafer may be, for example, silicon dioxide, silicon nitride, or other insulating materials. When a charged-particle beam impinges upon such an insulating sample, charges may build up in the sample due to the negatively-charged electrons being deposited on or embedded within the insulating layer, or due to a net positive charge which remains when secondary and/or backscattered electrons leave the surface, or combinations of these effects.
This charging of the sample can be detrimental to the performance of the charged-particle imaging or measurement system. For example, the charge distribution can create a net electrostatic potential on the film surface that will change the landing energy of the primary electron beam. Such changes in electrostatic potential are difficult to model quantitatively as they depend upon the film stack's resistance and capacitance which may strongly vary with process layer deposition and etching parameters. Uncertainty in the actual landing energy may adversely impact the performance of the system. For example, inaccuracies may be introduced into an ESX system's determination of film thickness or composition since the ESX system utilizes landing energy to predict characteristic x-ray production from the materials under measurement.
One conventional approach for reducing the impact of charging is to expose the sample with a beam from an alternate charged-particle source, typically an electron “flood gun.” This may be done to put the sample into a particular charge state, and to reduce or control the charging effects observed during subsequent measurements. However, this approach has various disadvantages and difficulties. For example, it may leave the sample in a charge state which depends on the history of the prior bombardment or treatment of the sample. Furthermore, the flood beam itself is subject to deflection and other effects discussed above. Finally, a flood gun is typically costly to add and difficult to locate in close proximity to or coincident with the primary charged-particle beam, or to operate simultaneous with the primary beam.
As discussed above, problems and difficulties are caused by charging of samples being examined in a charged-particle beam system. Hence, it is desirable to improve techniques for controlling sample charging in charged-particle beam systems.